Use diagnostic::on_unimplemented to make compile errors more helpful and less intimidating

[lloydmeta]

Right now, compile-time errors on transmogrify can be truly terrifying and unhelpful.

The new #[diagnostic::on_unimplemented] attribute from 1.78.0 might be helpful here.

That said, much of the functionality around LabelledGeneric and friends are built on top of smaller, more basic traits, which are based on other traits.... so I'm not really sure how far we can really go. Worth a think though.

https://github.com/lloydmeta/frunk/blob/1ca9c995b966ef204accd6b992401acfec302500/core/src/labelled.rs#L829-L911